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Hovertrekker
4-Sep-12, 09:45 PM
Hi all - Need some advice here.
I'm using a number 8 (3 bolt) centaflex coupling in my Raptor with a Rotax 793 twin.
When I pull start my engine, it always starts 2nd pull, but sort of burbles along at a low rpm below idle, making a lot of belt/transmission noise until I squeeze the throttle a few times and bring it up to a fast idle then it's all good. When I used to run the pulley directly off the crank in the last craft I never had this problem obviously. So I attribute it to the torsional resistance of the coupling counteracting the crankshaft. Is this normal - or do I need to be using a stiffer coupling like a number 12? I suspect that it's harming my belt and coupling to run like this.

What are you guys with Rotax 809's using? I've heard things about different engines needing to use a stiffer coupling. If anyone has any thoughts or experience with this, I welcome your input.

Thanks!

turnbu3656
5-Sep-12, 11:28 AM
Moving from direct drive to a coupling system introduces the springiness of the coupling into the system, and would reduce the mass on the PTO shaft, both of which would make the engine more liable to hunt at low speeds.

Sub idle torsional vibration was a regular problem with the big twins (we had it with the 400 twin so badly we put a small centrifugal clutch on it). I think it is a transmission resonance issue partly caused by the presence of the coupling, but also influenced by belt tension (tighter belt reduces the tendency) and basic engine charicteristics. It can put a lot of energy into the coupling, belt and fan, causing premature wear to all components, blade flap and can crack fan frames if allowed to continue.
The guys who used the 793s over here for any length of time often ended up using the 4 bolt (type 12) coupling in an attempt to reduce this tendency, but they also changed them on a regular basis as they wear out and drop to bits due to the harsh power delivery and vibration of the engine. I got rid of the 793 twin and started with the triples fairly early on (in retrospect a very good move). The 809 doesn’t suffer from the same issue as its much smoother and so you can use virtually any coupling. There’s no one using the 793s in the uk at the moment, all preferring the triple or 4 cylinder 4 strokes.
The big Polaris twins don't appear to suffer as badly as the rotax equivalent, but then they are all being used in a different environment from the original design intent where the mass of the huge varimatic centrifugal clutch on the PTO shaft damps out any tendency for the engine to hunt sub idle in the sled.

Regards Dan

Hovertrekker
5-Sep-12, 02:45 PM
Thanks for the feedback Dan! You've confirmed what I suspected about this engine. I initially went after the 793 thinking it would be lighter/smaller than the 809, less complicated with 1 pipe instead of 3 and 2 carbs instead of 3. So maybe I need to re-think the whole engine situation. I haven't seen anyone in the UK running a Rotax 600 twin (593) but I suspect they run smoother also. The Polaris 800's of Paul Hibbard and Chris Barlow seem to run fairly smooth from what I can remember from the worlds, but I don't really know for sure how they compare with the Rotax. I remember Chris saying he tried the Rotax once and it vibrated badly compared to the Polaris.

Hovertrekker
6-Sep-12, 04:01 PM
Dan - when you start your 809 does it hunt at all, or does it come right up to normal rpm? And what size coupling are you using?